This invention relates to an improved low pressure door closure device for providing a positive seal to process tubes wherein silicon wafers are heated and subjected to various chemical processings. The apparatus provides means for encapsulating a wafer paddle when the paddle is inserted into a process tube.
During the processing of microchips using the chemical vapor deposition (CVD) type system, maintaining a controlled environment is essential. It is essential that the process tube be kept absolutely clean. To accomplish this, the interior of the tube is cleaned and then evacuated. Next, silicon wafers are placed in a carrier or "boat", and then the boats themselves are placed on some type of instrument to introduce the boats to the processing tube. This instrument is normally called a "paddle". Paddles are utilized to hold and support the wafers while they are being processed inside the evacuated process tube. During this processing, the atmosphere must be controlled by first evacuating the tube completely and then entering certain desired gases. It is essential to assure that oxygen and other undesirable gases are prevented from entering the tube. The introduction of undesired gases during processing would ruin the processing and thus destroy the wafers. Presently, the state of the art concerning sealing the processing tube around the area where the paddle is introduced consists primarily of passing the paddle through a structure which meets the dimensions of the processing tube to seal it from the outside. The crucial element, however, is to create an effective seal around the shaft handle of the paddle. Conventional methods use compression, "O" ring, seals which are pressed against the shaft handle of the paddles. The seals are connected to the structure which seals the processing tube itself and thus a vacuum seal is created in the processing tube. There are several problems associated with this type of conventional door closure apparatus. The primary problem is that the extememe heat of processing is transferred through the paddle to the paddle shaft handle and thence to the "O" ring thereby causing the "O" ring to overheat and break down. Once the "O" ring on the paddle handle has burned, there is no assurance that a vacuum can be maintained and costly wafers may be lost inadvertently. The temperature of the process may range from between 600 and 800 degrees centigrade inside the processing tube and significantly high temperatures are also transmitted along the paddle handle.
A further drawback is that this type of compression "O" ring seal around the paddle handle is not practical with square paddle handles, of which there are a significant number. A further drawback is that loss of heat through the paddles causes the processing system to be less efficient than it could be if no such loss were allowed. It, therefore, is an object of this invention to provide an improved low pressure door closure apparatus and method for providing a positive seal about the handle of paddles such that compression "O" ring breakdown due to heat transfer is avoided.